1. Field of the Invention
The present invention relates to electronic convertors for low voltage filament lamp applications such as halogen lamps, and more particularly to auxiliary circuits for adding increased functionality to such convertors.
2. Related Art
Until now, almost all electronic convertors (often called electronic transformers) for low voltage filament lamp applications have up to now been based around self-resonating bipolar transistor half-bridge circuits. A new approach, using power MOSFETs driven by a control IC incorporating additional functionality tailored to this application, is realized in the IR2161 control IC and demonstrated in a typical application described herein.
The IR2161 is a dedicated intelligent half bridge driver IC for a halogen convertor (the word “convertor” being spelled in accordance with standard IEC 61047 “DC or AC supplied convertors for filament lamps—Performance requirements”) or “electronic transformer” targeted at medium and high end performance low voltage lighting applications. Considerable functionality has been incorporated within a low cost 8 pin DIP or SOIC package to allow reliability and performance advantages over existing circuit designs with a lower component count.
Electronic transformers are commonly used in place of wire wound step down transformers in order to provide the correct supply for widely used low voltage (generally 12V) filament lamps such as dichroic halogen lamps. Advantages are: (1) small size and weight, (2) fault protection circuitry, and (3) safety because of low output voltage.
Electronic transformers have become popular for low voltage lighting applications. The range of available products ranges from very small 50 W units, capable of driving only a single 50 W lamp, to 300 W units capable of driving up to 6×50 W lamps. In many applications the lamps are attached to a track system in which the supply rails are exposed. Since the voltage is only approximately 12V this does not present any safety problem.
The electronic transformer is generally smaller and lighter than a wire wound equivalent and may be equipped with short circuit and thermal protection, which are generally not included when a wire wound transformer is used. Generally a well-designed electronic transformer may be dimmed with a triac based leading edge phase cut dimmer (as can a wire wound transformer) or by a trailing edge transistor dimmer.
A block diagram of the IR2161 is shown in FIG. 1 and a typical application is shown in FIG. 2. These circuits have already been described in the above-mentioned Ser. No. 10/443,525 so that only a brief description need be presented here.
Referring to FIGS. 1 and 2, the IR2161 IC includes both oscillator and shut-down circuitry, including an additional thermal shut-down avoiding the need for external thermistors.
The IR2161 provides low and high side output drives HO and LO for the half-bridge MOSFETs or IGBTs M1 and M2. The output from the half bridge is connected to a high frequency stepdown transformer, which supplies approximately 12 Vrms at the output to drive the lamps. The IR2161 also incorporates all of the protection features needed in the system without the need for many external components.
At switch-on, the frequency sweeps from a high frequency around 125 kHz down to the normal operating frequency 30 to 40 kHz over a period of approximately 1 second. Because of the leakage inductance in the transformer this causes the output voltage at the lamp to start at a reduced value and gradually increase to the 12V nominal level. This reduces the inrush current at switch on. When the lamp is cold the filament resistance is lower which tends to cause high inrush currents that can cause false tripping of the shutdown circuit.
The IC includes a short circuit protection that operates if a high current is detected for approximately 50 msec causing the outputs to switch off. Similarly if an >50% overload is detected for more than approximately 500 msec then the outputs will switch off. It should be noted that the shutdown time under an overload condition will be reduced depending on the magnitude of the overload.
This dual mode shutdown circuit will protect the circuit from all output fault conditions and will function when the convertor is dimmed, and it will also protect the system from a short circuit at the end of the output leads.
An electronic transformer is normally required to provide a reasonably consistent output voltage over a range of loads and so the IC senses the load through the current sense resistor RCS and increases the frequency as the load is reduced thus providing compensation for the output transformer load regulation. There is also some modulation of the frequency through the line voltage half-cycle to spread the harmonics and reduce the size and cost of the EMC filtering components required.
The IR2161 includes all necessary protection features and also allows the convertor to be dimmed externally with a standard phase cut dimmer for leading or trailing edge. This IC provides the advantage of longer lamp life due to soft start and output voltage shift (load regulation) compensation. It causes the convertor to run with optimized harmonic behavior (i.e. almost unity power factor) also at higher loads (solution with bipolar devices can have harmonics problems for high loads due to the fact that the oscillator start-stop every half cycle, causing cross over distortion).
The IC also includes adaptive dead time to realize soft-switching and allow cool running MOSFETs (and improves the EMI behavior due to frequency modulation during the line voltage half cycle).
Some points to consider when comparing a halogen convertor circuit design with an electronic ballast for fluorescent lamps:    Filament Lamp is a Resistive Load    No Preheat/Ignition is required    DC Bus is full wave rectified line without smoothing    Close to Unity Power Factor is inherent in system    Can be dimmed with Triac (standard domestic type) dimmer    Dimming is achieved by PHASE CUTTING of the AC line    Output is isolated Low Voltage    Protection is provided against output short circuit or overload.    Shutdown is Auto-Resetting
The protection circuitry is auto-resetting so that if the output is short circuited the system will periodically try to restart and then shut down again. When the short-circuit is removed the lamp will be able to come back on again without the need for cycling the AC supply off and back on again to reset.
Of particular interest is the external capacitor at the CSD pin, which is used in several different operating modes, allowing the IC to be limited to 8 pins. The CSD pin is connected to different circuit blocks during different modes, controlled by internal control logic.
The current sense (CS) input is connected to the voltage compensation and shutdown circuit blocks. The CSD capacitor is switched between different circuits through internal transmission gates. The oscillator is voltage controlled and its input is connected to the CSD capacitor during normal operation. In case of overtemperature or external shutdown, the IC will go into a fault mode. In this mode the IC is in latched shutdown and will restart only after resetting the mains. In case of overvoltage or overload, the IC will go into an auto-resetting fault.
The different modes of operation of the IC and the system implications are shown in the state diagram of FIG. 3. FIG. 3 is a state diagram presenting the various operating modes of the IR2161, namely the UVLO mode, soft start mode, run mode, fault timing mode, fault mode, shut-down mode and standby mode. These operating modes are described further in Ser. No. 10/443,525.
Supplying VCC to the IR2161
The under-voltage lockout mode (UVLO) is defined as the state the IC is in when VCC is below the turn-on threshold of the IC. The IR2161 under voltage lock-out is designed to maintain an ultra low supply current under 300 μA and to guarantee the IC is fully functional before the high and low side output drivers are activated.
The capacitor (CVCC) is charged by current through supply resistor (RS) minus the start-up current drawn by the IC. This resistor charges CVCC to the UVLO+ threshold, at which point the IR2161 starts to operate and the LO and HO outputs become active. In a halogen convertor it is important to consider that the DC bus is completely unsmoothed and has a full wave rectified shape. CVCC should be large enough to hold the voltage at VCC above the UVLO threshold for one half cycle of the line voltage as it will only be charged at the peak.
An external 16V zener diode VZ has been added to avoid the need for the internal zener to dissipate power (it should be rated at 1.3 W). The resistor RD in series with CD enables the convertor to operate from a triac based (leading edge) phase cut dimmer. When the triac fires at a point during the mains half-cycle the high dv/dt allows a large current to flow through this path to rapidly charge CVCC to the maximum VCC voltage. In this way each line half cycle, the system will receive a fast pull up on VCC when the traic in the dimmer is fired. The external zener VZ will prevent possible damage to the IC by shunting excess current to COM. Once the capacitor voltage on VCC reaches the start-up threshold the IC turns on and HO and LO begin to oscillate.
A bootstrap diode (DB) and supply capacitor (CB) comprise the supply voltage for the high side driver circuitry. To guarantee that the high-side supply is charged up before the first pulse on pin HO, the first pulse from the output drivers comes from the LO pin. During under voltage lock-out mode, the high and low-side driver outputs HO and LO are both low.
Soft Start Operation
The soft start mode is defined as the state the IC is in at switch on of the system when the lamp filament is cold. As with any type of filament lamp the dichroic halogen lamp has a positive temperature coefficient of resistance such that the cold resistance (at switch on when the lamp has been off long enough to cool) is much lower than the hot resistance when the lamp is running. This normally results in a high inrush current occurring at switch on. Under worst-case conditions this could potentially trigger the shut down circuit. To overcome this problem the IR2161 incorporates the soft start function, shown schematically in FIG. 4.
When the IC starts oscillating the frequency is initially very high (about 125 kHz). This causes the output voltage of the convertor to be lower since the HF transformer in the system has a fixed primary leakage inductance that will present a higher impedance at higher frequency allowing less AC voltage to appear across the primary. The reduced output voltage will naturally result in a reduced current in the lamp which eases the inrush current, thus avoiding tripping of the shutdown circuit and easing stress on the lamp filament as well as high current in the half bridge MOSFETs (M1 and M2).
The frequency sweeps down gradually from 125 kHz to the minimum frequency over a period of around 1s (for a CSD capacitor of 100 nF). During this time the external capacitor at the CSD pin charges from 0V to 5V controlling the oscillator frequency via the internal voltage controlled oscillator (VCO). The value of CSD will determine the duration of the soft start sweep. However since it also governs the shut down circuit delays the value should be kept at 100 nF to achieve the correct operation.
Run Mode
When soft start is completed the system switches over to run mode. During this time the system provides some regulation of the output voltage of the convertor from minimum to maximum load. In this type of system it is desirable that the voltage supplied to the lamp does not exceed a particular limit. If the lamp voltage becomes too high the temperature of the filament runs too high and the life of the lamp is significantly reduced. The problem is that the output transformer is never perfectly coupled so there will always be a degree of load regulation. The transformer has to be designed such that the lamp voltage at maximum load is sufficiently high to ensure adequate light output.
At minimum load the voltage will consequently be higher and is likely to exceed the maximum desired lamp voltage. The load current is sensed via the current sense resistor (RCS). The peak current is detected and amplified within the IC then appears at the CSD pin during voltage compensation mode. The voltage across the CSD capacitor will vary from 0V if there is no load to approximately 5V at maximum load.
This is provided that the correct value of current sense resistor has been selected for the maximum rated load and line voltage supply of the convertor. This should be 0.33 Ohm (0.5 W) for a 100 W system running from a 220-240 VAC line. (It should be noted that the RCS resistor value sets the limits for the shut down circuit.)
In run mode the oscillator frequency will vary from approximately 30 kHz when VCSD is 5V (maximum load) to around 60 kHz when VCSD is 0V (no load). The result of this is that at lighter loads, for example if only a single 35 W lamp is connected to a 100 W convertor, the frequency will shift upwards so that the output voltage falls below the maximum that is acceptable for the lamp. This provides sufficient compensation for the load to ensure that the lamp voltage will always be within acceptable limits but does not require a complicated and expensive system involving feedback from the output.
An additional internal current source has been included to discharge the external capacitor. This will provide about 10% ripple at twice the line frequency if CSD is 100 nF. See FIG. 5.
The advantage of this frequency modulation (or “dither”) is that during the line voltage half cycle the oscillator frequency will vary by several kHz, thus spreading the EM conducted and radiated emissions over a range of frequency and avoiding high amplitude peaks at certain frequencies. In this way the filter components used may be similar to those used in a common bipolar self-oscillating system.
FIGS. 6A and 6B show the VCO frequency ranges as a function of temperature in running mode and in soft start mode, respectively. Frequency variations over temperature have been minimized. Acceptable linearity exists in run mode, whereas in soft start mode it is not needed. Minimum frequencies are the same in two modes.
Shut Down Operation
The IR2161 contains a dual mode auto-resetting shutdown circuit (FIG. 7) that detects either a short circuit or overload condition at the output. The load current detected at the CS pin is used to sense these conditions. If the output of the convertor is short-circuited, a very high current will flow in the half bridge and the system must shut down within a few mains half cycles otherwise the MOSFETs will rapidly be destroyed due to excessive die temperature. The CS pin has an internal threshold of approximately 1.0V so that if the voltage exceeds this level for more than 50 msec the system will shut down.
A delay is included to prevent false tripping either due to lamp inrush current at switch on (this current is still higher than normal with the soft start operation) or transient currents that may occur if an external triac based phase cut dimmer is being used.
There is also a lower threshold of 0.5V, which has a much longer delay before it shuts down the system. This provides the overload protection if an excessive number of lamps is connected to the output or the output is short-circuited at the end of a length of cable that has sufficient resistance to prevent the current from being large enough to trip the short circuit protection.
Under this condition there is also an excessive current in the half bridge that is sufficient to cause heating and eventual failure but over a longer period of time. The threshold for overload shutdown is approximately 50% above maximum load with a delay of approximately 0.5s. This is based on a current waveform that has a sinusoidal envelope with a high frequency square wave component with 50% duty cycle.
Both shutdown modes have auto reset, which allows the oscillator to start again approximately 1s after shutting down. This is so that if the fault condition is removed the system can start operating normally again without the line voltage having to be switched off and back on again. It also provides a good indication of overload to the end user as all the lamps connected to the system will flash on and off continuously if too many are connected.
The shut down circuit also uses the external CSD capacitor for its timing functions. When the 0.5V threshold is exceeded at CS the CSD is internally disconnected from the voltage compensation circuit and connected to the shutdown circuit. The oscillator operates at minimum frequency when the CSD capacitor is required for shutdown circuit timing. When the 0.5V threshold is exceeded the IR2161 charges CSD rapidly to 4V.
When the shutdown threshold of 0.5V is exceeded the CSD capacitor is charged by current source I_OL and when the threshold of 1.2V is exceeded it is charged by I_SD as well. If 1.2V is exceeded CSD will charge from 4V to 12V in approximately 50 ms. When 0.5V is exceeded but 1.0V is not, CSD charges from 4V to 12V in approximately 0.5s. The timing accounts for the fact that high frequency pulses with approximately 50% duty cycle and a sinusoidal envelope appear at the CS pin. The values of I_SD and I_OL take into account that only at the peak of the mains will the comparator outputs go high and effectively the capacitor will be charged in steps each line half cycle.
If a fault is detected but disappears before CSD reaches 12V then CSD will discharge to 2.5V and then the system will revert to compensation mode without interruption of the output.
Similarly when the system starts up again after a delay the CSD capacitor will be internally switched back to the voltage compensation circuit. If the fault is still present the system will immediately switch CSD back to the shutdown circuit.
The IR2161 can be shut off by applying a voltage above 0.5 VDC to the CS pin. This will cause the system to go directly to fault mode after approximately 1 μS such that it is necessary for VCC to be re-cycled off and on to restart the system.
The IR2161 also includes over-temperature shutdown, which latches the convertor off when the die temperature of the IC exceeds 130-140° C. It is assumed that the die temperature will be approximately 20° C. above the ambient temperature inside the convertor. Depending on layout, heat will be transferred from other devices through the PCB traces into the IC, raising the temperature. This behavior may cause the IC to shut down if high temperatures from the MOSFETs are conducted to the IC.
Calculating RCS
To achieve effective operation in the IR2161 based halogen convertor, the value of the current sense resistor RCS is calculated as follows (see FIG. 8).
Ignoring the output transformer we can assume for this calculation that the load is connected from the half bridge to the midpoint of the two output capacitors and that the voltage at this point will be half the DC bus voltage. The RMS voltage of the DC bus is the same as that of the AC line so we can see that the RMS voltage across the load shown in FIG. 8, will be half the RMS voltage of the line. The load is the maximum rated load of the convertor. The current in Rcs will be half the load current given by:
      I          CS      ⁡              (        RMS        )              =            P      LOAD              V      AC      
Since the load is resistive the current waveform will have a sinusoidal envelope and so the peak can be easily determined taking into account that the current is has a high frequency component with an approximate 50% duty cycle:ICS(PK)=2√{square root over (2)}×ICS(RMS)
Therefore:VCS(PK)=ICS(PK)×RCS
For correct operation at maximum load the peak voltage should be 0.4V. The calculation can be simplified by combining the formulae,
      R    CS    =            0.4      ·              V        CS                    2      ·              2            ·              P        LOAD            
Which can be simplified to:
      R    CS    =      0.141    ·                  V        AC                    P        LOAD            